1. Field of the Invention
The present invention relates to a NOx (i.e., nitrogen oxides) sensor abnormality detection device for detecting an abnormality in a NOx sensor provided downstream of a NOx purification device and to an exhaust gas purification system using the NOx sensor abnormality detection device.
2. Description of Related Art
Conventionally, there is a publicly known exhaust gas purification system that has a NOx sensor provided downstream of a NOx purification device, which purifies NOx in exhaust gas, for sensing a NOx concentration and that determines NOx purification quantity of the NOx purification device based on an output signal of the NOx sensor (for example, as described in Patent document 1: JP-A-2002-47979).
Such the exhaust gas purification system is required to detect an abnormality in the NOx sensor due to degradation, a failure and the like in order to accurately sense the NOx purification quantity of the NOx purification device.
The technology of Patent document 1 determines the abnormality in the NOx sensor based on an estimated NOx concentration and the NOx concentration sensed with the NOx sensor in a specific state where the NOx concentration in the exhaust gas can be estimated. For example, the technology of Patent document 1 detects the abnormality in the NOx sensor by determining whether the output signal of the NOx sensor is a value equivalent to zero of the NOx concentration in an operation state where a fuel cut state occurs and NOx emission quantity from the internal combustion engine is estimated to be zero.
A deviation of the NOx concentration, which is sensed based on the output signal of the NOx sensor in an abnormal period when a gain deviation is caused in the output of the NOx sensor and an inclination of an output characteristic of the NOx sensor has changed, from the NOx concentration sensed based on the output signal of the NOx sensor when the NOx sensor is normal reduces as the NOx concentration sensed with the NOx sensor reduces.
Therefore, if the abnormality in the NOx sensor is detected by determining whether the output signal of the NOx sensor is a value equivalent to zero of the NOx concentration in the operation state where the NOx concentration is estimated to be zero as the specific state where the NOx concentration can be estimated in Patent document 1, it is difficult to distinguish between the normality and the abnormality of the NOx sensor.
Moreover, when the gain deviation and an offset deviation are caused in the output of the NOx sensor, in some cases, the estimated NOx concentration in a specific state other than the zero point where the NOx concentration can be estimated coincides with the sensed NOx concentration sensed with the NOx sensor. As a result, there is a possibility that the abnormality in the NOx sensor cannot be detected although the gain deviation and the offset deviation are caused in the output of the NOx sensor.